This invention relates generally to nozzle vanes for use in gas turbine engines and, more particularly, to improved sealing means therefor.
It is well understood that the performance of a gas turbine engine turbine can be enhanced by incorporating a variable area turbine nozzle, a stage of variable position vanes which controls the flow of hot combustion gases into the downstream rotating turbine rotor blade row. Such turbine nozzle variability is necessary in advanced variable cycle engines in order to obtain variable cycle characteristics since the propulsive cycle balances out differently as the turbine nozzle area is changed. One characteristic of nozzle vanes which presents a difficulty is that they are disposed in proximity with circumscribing shrouds. However, since the variable area nozzle vane must be able to rotate open and closed to regulate nozzle area, it cannot be rigidly attached to these shrouds. As a result, one of the major concerns in the design of such variable area turbine nozzles is what is commonly referred to as "end wall leakage" or the flow of turbomachinery operating fluid from the vane airfoil pressure surface to the suction surface through the gap between the end of the nozzle vane and its associated proximate shroud. Since turbine efficiency decreases with increasing vane end clearance, it is desirable to minimize the clearance to maximize efficiency. However, some gap is required to preclude undesirable frictional contact between the vane end and shroud because the plane of rotation of the moving vane is not exactly true. Also, large swings in temperature of the operating fluid entering the turbine cause variations in clearance which must be accounted for. These problems have long been recognized and many types of floating seals have been proposed to minimize this end wall leakage. However, in most of these designs the nozzle sidewalls combine to form an open end or cavity in the vane in which the seal floats, urged into promixity with the circumscribing shroud by gas pressure being provided from within the vane. As a result of the vane cavity being enclosed by the sidewalls, a portion of the trailing edge remains unsealed, allowing operating fluid to leak across that portion of the vane end and adversely affecting turbine nozzle efficiency. Furthermore, in most designs, even if the seal and its associated cavity were to extend to the vane trailing edge, the usual source of high pressure internal vane cooling air could not be utilized to hold the seal trailing edge into contact with the shroud since this pressurized air could not be routed to that portion due to the thinness of vane trailing edge. It becomes desirable, therefore, to have a floating vane end seal which extends entirely to the vane trailing edge and which may be urged into contact with the proximate shroud along its entire length to minimize end wall leakage.